220 CHEMICAL ENGINEERING
be put through a given machine, only a certain number of layers of particles can be treated at one time on a given surface, which explains the small capacity of fine concentrating machines. Based on surface, the distribution of feed to different machines can be made more efficiently. Also the character of the crushing operations can be modified to give the largest reduction possible with the minimum. production of useless fine surface.
The two diagrams, Figs. 15 and 16, are made from the logsheets of the tests, all records of crushing pressures being plotted directly on the sheet against the movement of the faces of the machine. The triangular curved areas are the record of the work on the machine, and are a measurement of energy. Each crushing movement was continued until the pressure was rapidly going up with little movement of the faces, then the pressure was gradually reduced and we got the return curve showing that some energy was being given up by the rock. The machine was then opened, the finer sizes screened out and the coarse returned and crushed again. After the crushing was complete as far as was considered advisable, the screen analysis was made and plotted as the weights were determined. One hundred grams was used it each case, its original screen analysis being the first thing plotted on the paper.
Without going into the calculations in detail, the energy absorbed in the case of Fig. 15 was 166.3 ft.-lb., while in the case of Fig. 16, the energy absorbed was 295.6 ft.-lb., about 80 per cent more. And yet the area of the surface under 200 mesh was greater in the case of the former. The only way to account for the energy is to go beyond the 200-mesh line, and doing this, using logarithmic paper and assuming that the curve continues its law indefinitely among the smaller particles, I find that out to 5,000 mesh for Fig. 15 the energy is equal, per unit area, to that in Fig. 16 out to 11,000 mesh. Both are calculated to have one-half of 1 per cent at those sizes.
Screening and sizing of the products from the various grinding machines will be found treated in the following section.